1. Field of the Invention
The present general inventive concept relates to an image forming apparatus. More particularly, the present general inventive concept relates to a fixing device that fixes a toner image onto a printing medium, and an image forming apparatus having the fixing device.
2. Description of the Related Art
Image forming apparatuses, such as printers, copiers, scanners, multi-function machines, and the like, include fixing devices which fix developer images, such as toner images, which are transferred onto sheets of paper by transferring devices, which are widely known to those skilled in the art.
In order to meet the recent demand for high-speed image forming apparatuses, the ability to rapidly increase a temperature and improved fixing properties are required in fixing devices. In order to rapidly increase the temperature, a thermal capacity of heating units in fixing devices needs to be reduced so a period of time required to raise the temperature can be reduced. Additionally, in order to improve fixing properties thereof, there is a need for a structure in which a width of a nip zone may be increased and pressure may effectively act on toner images so that heat from heating units can be effectively transferred to toner images.
FIGS. 1A and 1B are sectional views schematically illustrating conventional fixing devices. FIG. 1A illustrates a conventional roller-type fixing device, and FIG. 1B illustrates a conventional belt-type fixing device.
The conventional roller-type fixing device of FIG. 1A includes a pressing roller 10 and a heating roller 20 which rotate while tightly in contact with each other, and a heating unit 30, for example a heater, which is mounted in the heating roller 20.
The conventional roller-type fixing device configured as described above applies heat and pressure onto a non-fixed toner image T transferred onto a surface of a sheet of paper P by a transferring device known to those skilled in the art while the sheet of paper P passes through a nip zone N formed by pressure contact between the pressing roller 10 and the heating roller 20, which rotate in contact with each other, and then fuses the toner image onto the sheet of paper P. The conventional roller-type fixing device provides benefits for high-speed printing, because the temperature decrease when feeding sheets of paper is relatively small. However, since the heating roller 20 has a large thermal capacity and applies heat not only to the nip zone N but also to the entire surface of the heating roller 20, a long period of time is required to raise the temperature. Additionally, the nip zone N is formed in a contact area between the pressing roller 10 and the fixing belt 20, and thus the width of the nip zone N may be reduced, making difficult to implement the shape of the nip zone N in a variety of forms.
Referring to FIG. 1B, the conventional belt-type fixing device includes a pressing roller 10, a fixing belt 40 which is made to rotate by a rotation force transferred from the pressing roller 10, a guide member 50, which is mounted inside the fixing belt 40, to guide the rotation of the fixing belt 40, and a heating unit 60, which is mounted on the guide member 50, to heat a nip zone N of the fixing belt 40.
In the conventional belt-type fixing device configured as described above, since the heating unit 60 has a small thermal capacity and applies heat to only the nip zone N, it is possible for the period of time required to raise the temperature to be less than in the conventional roller-type fixing device of FIG. 1A. Additionally, a width of the heating unit 60 is increased, and thus it is possible to increase the width of the nip zone N. However, as the heating unit 60 simultaneously applies heat and pressure to the nip zone N to fix the toner image T on the printing medium P, the pressurizing force may be limited by the durability of the heating unit 60. Therefore, a great pressurizing force does not act on the nip zone N, and the pressurizing force may not be sufficient to fix the image. Additionally, if an excessive pressurizing force is applied to the nip zone N in order to improve the fixing properties, the heating unit 60 can be broken due to the pressurization and thermal deformation.